1. Field of the Invention
The present invention relates to a radiation imaging apparatus, and in particular, to a radiation imaging apparatus for generating image data by using correction data.
2. Related Background Art
Various types of imaging apparatuses have been produced as in the past, such as those using Si single-crystal sensors represented by a CCD-type sensor and a MOS-type sensor and large-size sensors having imaging devices comprised of PIN-type sensors of hydrogenated amorphous silicon arranged one-dimensionally or two-dimensionally.
As for such imaging apparatuses, they are not only used to obtain visible optical images as represented by a digital camera and a digital copying machine, but also development thereof as radiation imaging apparatuses for converting a radiological image into an electrical signal is underway in conjunction with advance in nuclear development, radiation medical instruments or nondestructive inspections.
There are many cases where an S/N ratio of the aforementioned imaging apparatuses is two to three digits, and the S/N ratio exceeding that has not been demanded as in the past. It has the following reasons (1) and (2) for instance.
(1) There is no analog-digital (A/D) converter suited to digitizing an output of a high S/N ratio with high precision.
(2) A data amount after A/D conversion becomes such a large capacity that restrictions are put on a memory and communication, resulting in inferior usability.
In recent years, however, there is increasing necessity of the radiation imaging apparatus having a high S/N ratio of four to five digits because of improved performance of the A/D converters, enlarged capacity of the memory and the advance in high-speed communication technology.
Nevertheless, the aforementioned imaging apparatuses in the past cannot avoid reduction in the S/N ratio due to dark noise because of variations in production processes.
Thus, the following method is proposed, for instance, as a method for solving the problem of the reduction in the S/N ratio.
First, on factory shipment of the radiation imaging apparatus, the apparatus has correction data for correcting the noise caused by a dark current (hereafter, referred to as dark noise) stored in the memory. And when the radiation imaging apparatus is used in reality, an image data obtained by imaging an object is corrected by using the correction data in said memory.
However, such a method has the problem described below.
First, in the case where a user obtains the image of the object by using the radiation imaging apparatus, the user usually selects and sets an operating condition for the apparatus based on the object, circumstances thereof, purposes of the imaging and so on.
In this case, each component constituting the radiation imaging apparatus changes its characteristics according to temperature and so on. Therefore, the condition on actually performing the aforementioned imaging is different from the condition on obtaining the correction data (factory shipment), and the dark noise as a cause of the reduction in the S/N ratio is subtly different in conjunction with it.
For this reason, even if the image data obtained on the actual imaging is corrected by using the correction data, an error included in the image data is not completely corrected. In particular, as disclosed in Japanese Patent Application Laid-Open No. 2001-268440, even if a correction is made by preparing offset data (correction data) for each imaging time, no correction can be made as to the error due to a factor other than the imaging time such as temperature change.
As previously described, it is a serious problem that the condition on the actual imaging is different from the condition on obtaining the correction data when obtaining imaging data of the high S/N ratio.
Thus, as disclosed in Japanese Patent Application Laid-Open No. 2001-141832, or Japanese Patent Application Laid-Open Nos. H10-208016, H10-327317 and H11-151233 by the assignee hereof, there is a proposed method of performing the same operation as the actual imaging in close timing to the imaging and obtaining the correction data in order to obtain the image data of the high S/N ratio by obtaining the correction data on approximately the same condition as the actual imaging.
However, there is a possibility of losing imaging timing desired by the user for the sake of obtaining the correction data.
Thus, as for the aforementioned technology, the image data is output through a correction process after waiting until the correction data is obtained and both the image data and correction data are completely obtained. Therefore, it takes time from an instruction for the imaging provided by the user until an actual output of the (corrected) image data.
As the correction data is obtained on the same condition as the actual imaging, delay time until a data output increases in proportion to exposure time on the imaging.